There is an increasing demand for battery systems which are intended to be used in stationary applications such as wind power installations and standby power supply systems, or else in vehicles. All of these applications place stringent requirements on reliability and failsafety. The reason for this is that complete failure of the voltage supply through the battery system can lead to a failure of the entire system. For example, in wind power installations, batteries are used in order to adjust the rotor blades when the wind is strong and to protect the installation against excessive mechanical loads which could damage or even destroy the wind power installation. If the battery in an electric car were to fail, the car would become undrivable. A standby power supply system is in turn intended precisely to ensure interruption-free operation, for example of a hospital, and therefore cannot itself fail, as far as possible.
In order to make it possible to provide the power and energy required for the respective application, individual battery cells are connected in series, and in some cases additionally in parallel. FIG. 1 shows an outline circuit diagram of batteries connected in series. A multiplicity of battery cells 10-1 to 10-n are connected in series in order to achieve the high operating voltage, as required for the electric motor in a passenger car, for example, by addition of the voltages of the individual cells 10-1, . . . , 10-n. The high operating voltage can be decoupled by output-side switches 11-1 and 11-2 from the downstream power-electronic components, such as inverters, which are not illustrated. Since the total output current of the battery flows in each of the battery cells 10-1, . . . , 10-n because the battery cells 10-1, 10-n are connected in series, with the charge transport taking place by electrochemical processes within the battery cells 10-1, . . . , 10-n, the failure of a single battery cell means, in the extreme, that the entire arrangement can no longer provide any current and therefore any electrical energy. In order to allow a threat of failure of a battery cell 10-1, . . . , 10-n to be identified in good time, a so-called battery management system 12 is normally used, which is or can be connected to both terminals of each of the battery cells 10-1, . . . , 10-n and determines operating parameters such as the voltage and temperature of each battery cell 10-1, . . . , 10-n and, therefrom, their state of charge (SoC) at regular or selectable intervals. This means a high level of complexity with at the same time little flexibility for the electrical operating data of the battery system.
Further disadvantages of connecting a multiplicity of battery cells in series are as follows:
1. Conditions are imposed for the operating voltage to be provided, the maximum current and the stored energy for various operating states of the device to be operated using the battery, which conditions can be combined only when a greater number of battery cells are coupled than would actually be necessary to comply with the individual requirements. This increases the price, as well as the weight and volume of the battery system, which are particularly disruptive in the case of an electric car.
2. The installation of the battery, that is to say the interconnection of the individual cells, is performed at high voltages up to 1000 V, because the voltages of the individual battery cells are added by being connected in series, as a result of which the battery, individual cells or modules cannot be replaced in local workshops or, in the case of stationary use, can only be replaced using a special tool by a specially trained skilled workman. This results in a high degree of complexity in terms of logistics for maintenance of battery systems in the event of a fault.
3. In order to connect the battery system to be free of voltage, that is to say to disconnect the actual battery from the load, circuit breakers 11-1 and 11-2 need to be provided, which are typically in the form of contactors, and are very expensive for the high currents and voltages to be expected.